This invention relates to optical transmitters, and in particular to optical transmitters incorporating a plurality of laser devices.
Optical communications networks are now conventional in long distance communications networks and are becoming more commonplace for inter-office communication networks, LANs, etc. Typically, these networks tend to be characterised by having a high density of short-length network links.
FIG. 1 shows a schematic depiction of a conventional laser driver for use in an optical transmitter (or combined optical transmitter/receiver). Driver control unit 10 comprises control logic 12, first and second digital-analogue converters (DACs) 14, 15 and first and second analogue-digital converters (ADCs) 16, 17. The driver control unit is connected to the transmitter unit 20, which comprises laser diode 22 and monitor system 24. The monitor system is connected to the second DAC via a low pass filter (LPF) 32 and power monitor 34.
In use, the control logic 12 receives data from a network component (not shown) and determines the signals that need to be sent to the laser diode 22 in order for the appropriate optical signal to be launched into optical fibre 40. This is achieved by sending a bias current, which controls the state of operation of the laser diode, and a modulation current, which causes the laser to emit appropriate pulses to form the desired ‘0’ and ‘1’ optical signals. The bias current signal and the modulation current signal are sent to first and second DACs 14, 15 respectively and the analogue currents are then sent to the laser diode.
The laser diode 22 emits an optical signal in accordance with the bias and modulation currents into the optical fibre 40. The monitor system 24 detects this optical signal and generates an electrical signal that is indicative of the optical signal launched into the optical fibre. A portion of this electrical signal is fed through a low pass filter 32 and then to the first ADC 16, the output of which is connected to the control logic. The control logic can interpret the digitised output of the LPF to determine the power output of the laser diode. The other portion of the electrical signal is passed to a power monitor, the output of which is connected to the second ADC 17. The control logic uses this signal to monitor the modulation and the extinction ratio of the laser. The control logic uses these feedback inputs to vary the bias and modulation current signals to obtain the desired optical desired signals.
The drive arrangement shown in FIG. 1 is required for each laser transmitter used in a communications network. It will be readily appreciated that as the number of laser transmitters used in a network grows, there will be issues associated with the space required for the driver circuits, the amount of electrical power required to operate them, the amount of heat that must be dissipated from the equipment and the cost of the driver circuitry.